1. Field of the Invention
This invention relates to new and improved methods of and apparatus for tuning a birefringent optical filter and, in particular, for tuning a Solc type birefringent optical filter. Accordingly, it is a general object of this invention to provide new and improved methods and apparatus of such character.
2. General Background
Optical frequency or wavelength selective filters with exacting bandpass characteristics have been used for the isolation of spectral lines. As the optical spectrum is being used for communications channels, such filters are being adapted for the separation of optical signals from a single transmission medium such as an optical fiber. Birefringent optical filters are capable of such exacting separation of signals having closely spaced wavelengths, and their design has progressed to allow the realization of any desired periodic transmission function as well as the simple sinusoidal response obtainable with a single birefringent crystal element located between polarizers. The theory and art of such birefringent optical filters have been reviewed in some detail by A. M. Title and W. J. Rosenberg in their article, "Tunable Birefringent Filters", Optical Engineering 20 (6), pp. 815-823 (1981).
A type of birefringent filter by Ivan Solc is discussed by A. M. Title et al. in their article supra citing as a reference Solc, I., J. Opt. Soc. Am. 55, p. 621 (1965). Their article also discusses the B. Lyot filter.
Also of interest is "Electro-optically tuned spectral filters: a review", W. J. Gunning, Optical Engineering November/December 1981/Vol. 20 No. 6, pp. 837-845. Seventy eight articles are cited therein including I. Solc, "A New Type of Birefringent Filter", Czech. J. Phys. 4, 53 (1954), B. Lyot, "The Birefringent Filter and its Application in Solar Physics", Ann. Astrophys. 7,31 (1944), and J. W. Evans, "Solc Birefringent Filter", 48, 142, (1958).
The Solc filter is discussed by J. F. Lotspeich et al., "Electro-optical tunable filter", Optical Engineering November/December 1981, Vol. 20 No. 6, pp. 830-836, citing therein, Ivan Solc, 55(6), 621 (1965).
Wave plate tuning is also discussed by John W. Evans, "The Birefringent Filter", J. Optical Soc. Amer. March 1949, Vol. 39 No. 3, pp 229-242.
The Lyot-type filter consists of a serial arrangement of several single element filter sections each having a different length birefringent crystal element between polarizing filters. Each section has a sinusoidal transmission versus frequency characteristic with a different periodicity, and only those wavelengths passed by all sections reach the filter output. All the remaining light is absorbed in the several interleaving polarizers. In 1953, Ivan Solc showed that it was possible to build a birefringent filter having a periodic pulse type of narrow passband by using a series of equal length birefringent elements oriented at different angles between a single pair of input and output polarizers. The design of this Solc-type lossless sequence of elements between a single pair of polarizers was further analyzed by Harris et al., (J. Opt. Soc. Amer, 54(10), 1267-1278 (1964). They showed that it was possible to synthesize with this lossless Solc-type configuration any desired periodic transmission characteristic to a degree of accuracy dependent upon the number of equal length birefringent crystal elements used. An advantageous feature of this lossless Solc-type birefringent filter is that after passing through the input polarizer, no light is lost before the output polarizing filter. Therefore, by simply replacing this output filter with a polarizing beam splitter, both the transmission bandpass and its complement can constitute useful outputs. Such a splitting of an optical signal into two complementary outputs is the required function of an optical frequency demultiplexer, a device useful in fiber optic communications systems.
In constructing such a Solc type filter, it is desirable to prepare the birefringent crystal elements with identical lengths; otherwise, the shape of the transmission function degrades. The optical frequency periodicity depends on the crystal length, but the exact crystal length also determines the phase of the period, that is, the tuning of the passbands and stopbands in the frequency domain. Some adjustable tuning is usually required, however, and, disadvantageously, this adds to the complexity of a practical Solc type filter.